(1) Field of the Invention
The present invention relates to a polygon mirror, and more particularly relates to a polygon mirror which is produced by resin-forming and is used especially for laser printers, plane paper facsimiles, and bar cord readers, etc.
(2) Description of the Related Art
Polygon mirrors for use in the laser scanning optical system used, for instance, in laser printers, etc. have been produced by lapping and polishing optical glass, or by performing super-fine cutting of a metallic material such as aluminum alloys etc. using a diamond cutter.
A laser printer employing such a laser scanning system is generally constructed as shown in FIG. 1.
Specifically, FIG. 1 is a perspective view showing a scanning optical system in a laser printer. The system is provided with a semiconductor laser oscillator 111 for generating laser light, a scanner controller 112 for turning on and off the laser oscillator 111, a collimator lens 113 for forming the laser light emitted from the oscillator 111 into collimated beams, a polygonal mirror device 114 reflecting the collimated laser beams from the collimator lens 113 depending upon revolving position, a f-.theta. lens 116 for forming a fine laser spot on a photoreceptor 115 from the collimated beams reflected on and from the polygonal mirror device 114.
In the above configurations, a polygon 117 of the polygonal mirror device 114, which is rotated at a high speed by a driving motor, deflects a laser beam LA in dependence upon its position, thus performing scanning in the longitudinal direction of the photoreceptor 115. In this while, the semiconductor laser oscillator 111 is modulated, to thereby create, a desired latent image on the photoreceptor 115, and the thus formed latent image is developed by the electrophotographic process to make a record on a printing sheet 119.
In the figure, reference numerals 120 and 121 denote a charger and a horizontal synchronization sensor, respectively. In recent years, an attempt has been made to manufacture polygon mirrors using resin moulding technique.
FIG.2 is a side view showing an example of mounting state of a polygon mirror having a conventional structure. Here, a polygon mirror 1 is mounted on a mirror mounting table 8 and pressed from the side of a clamping member-receiving surface toward the mirror mounting table 8 by a clamping member 9. As both the surface of the mirror mounting table 8 and the referential surface for attachment of the polygon mirror 1 in contact with the mirror mounting table 8 have an irregularity although being minute as in a level of microns, clamping stress A induces reaction force B, C, or etc., whose position cannot be expected. Accordingly, moment D and E occur, and cause a deformation although being minute as in a level of microns as shown in a broken line, particularly when a polygon mirror is made of a resin material which does not have as a large elasticity as metals. In the figure, the deformation shown by the broken line is depicted as if the polygonal member ate into the mounting table, but this is to show clearly that the polygon is deformed in the direction, and does not intend actual intrusion.
Since the light reflected by the mirror is laser light, this requires that the flatness of the mirror surface be within 1/3 to 1/8 .lambda. (wavelength of the laser light). Accordingly, if the above deformation occurs, the property of the reflected light is likely to change. In addition, each facet of the polygon is locally deformed so that the facet inclination error (the deviation from orthogonality of each mirror facet to the referential flat surface of the polygon mirror itself), is deteriorated.
Prior art polygon mirrors are attached in the following manners.
Japanese Utility Model Application Laid-Open Sho 59 No. 156216 discloses a fixing structure which comprises a mounting table securely fixed to a rotary shaft; and a rotational polygon mirror with a central hole, having a lager diameter than that of the rotary shaft and having a space for receiving a center adjuster thereinside on the side opposite the mounting table; and wherein the polygon mirror is coaxially positioned to the rotary shaft using a central adjuster.
A Japanese Patent Publication Sho 58 No. 15650 discloses a method of pressingly fixing a rotational polygon mirror used for a deflector deflecting laser beams and other light beams onto an attachment surface and conducting adjustment. But this structure needs a lot of parts for the pressing means, and to make matters worse, the parts are complicated, so the assemble takes labour and time thus increasing the cost.
On the other hand, in place of using a clamping member 9 as shown in FIG. 2, a polygon mirror may conceivably be fixed as shown in FIG. 3 which exemplifies another side view of a different prior art attachment of a polygon mirror. In this case, the polygon mirror 1 is fixed by forming a layer of an adhesive 10 between a referential surface 5 for attachment of the polygon mirror 10 and a mirror mounting table 8 on a polygon driving motor 7. However, if the layer of adhesive 10 cannot be spread out uniformly, the unevenness of the layer of adhesive 10 would rather degrade the precision of the orthogonality of facets to the referential surface. In addition, there is another fear that the mirror facets may be stained while the polygon is fixed.
Another Japanese Utility Model Application Laid-Open Hei 1 No. 160416 proposes a method of positioning and fixing. In this structure, center-positioning and fixing of a polygon is effected such that a projection formed in the center of the rotational polygon mirror is engaged with a hollow provided in a corresponding position of a mirror mounting table, with an adhesive applied on the interface between the projection and the hollow. It is true that this fixing method can provide a more stable adhesion than the former method shown in FIG. 3 in which the flat surface of the polygon mirror 1 is simply cemented with the surface of the mirror mounting table 8. But the method only aims at regulating the center of rotation, therefore, the measure against deformation of the mirror is, certainly considered to a slight degree, but is not sufficient.
As what has been discussed heretofore, resin material has a smaller elasticity as compared with metallic materials, so a polygon mirror made of a resin material is easy to deform, that is, surface inclination of the mirror facets is increased and the flatness of the mirror facets are degraded by fixing stress caused by the attachment of the polygon to the motor for rotating. This has been a great obstacle to put the resin polygon into practice.
More specifically, combination of the fixing stress and its reacting force generates moment causing a micron-order deformation of the mirror, and the moment deforms the mirror facets.